Titanium and titanium alloys are very reactive metals. They react with most of the halogenated and some non-halogenated reagents used in depositing hard, wear, abrasion and erosion protective coatings by a known deposition techniques such as chemical vapor deposition (CVD), CVD-like processes such as plasma-assisted CVD and physical vapor deposition (PVD) processes such as sputtering, plasma spraying and reactive ion plating. Because of their reactivity to halogenated reagents, it is difficult to chemically vapor deposit hard protective coatings that strongly adhere to titanium or titanium alloys. This is true because the halogenated reagents and their reaction products in the CVD and CVD-like processes react with the titanium and titanium alloys, causing spalling of the deposited coating. In the case of PVD processes, stresses due to the mismatch of the coefficients of thermal expansion can lead to poor adhesion and spalling.
Since noble materials such as cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver and gold do not react with halogenated reagents, it is conceivable to plate titanium and titanium alloys first with one of the noble materials and then coat them with hard protective coatings using known chemical and physical vapor deposition techniques. In fact, U.S. Pat. No. 4,427,445 peripherally discloses a process of depositing hard fine-grained tungsten and carbon alloy on titanium. It discloses that it is preferable to coat the substrate first by electrochemical deposition of a more noble material such as nickel or copper when depositing tungsten and carbon alloy on it.
Unfortunately, in the case of titanium and titanium alloys, normal electroplating procedures for noble materials do not yield an adherent deposit. This is because of the fact that titanium and titanium alloys rapidly form a tenacious oxide film in the presence of air and moisture. This oxide film is extremely difficult to remove by normal pre-plating and electroplating techniques. Additionally, this oxide film reforms very quickly as a result of a reaction between titanium and titanium alloys and water or solutions used in pre-plating and electroplating baths.
Because of the above problems with depositing adherent noble materials by normal electroplating techniques, special processes have been developed for preparing titanium and titanium alloys for electroplating. Three such processes for preparing titanium and titanium alloys for electroplating are discussed in the American Society for Testing and Materials (ASTM) Standard No. B481 entitled "Standard practice for Preparation of Titanium and Titanium Alloys for Electroplating." These methods describe that the adhesion of the electrodeposit is mechanical and, for this reason, it may be less than adequate. To improve adhesion, these methods suggest using nickel as an intermediate coating and heat treating it in an inert gas atmosphere for 1 to 4 hours at 540.degree. to 800.degree. C. The heat treatment causes interdiffusion of the nickel and titanium and produces a metallurgical bond.
The heat treatment of titanium and titanium alloys in a temperature ranging from 540.degree. to 800.degree. C. even for 1 hour, however, is undesirable from the standpoint of the mechanical strength of titanium and titanium alloys. This temperature range is close to both aging and heat treating temperature of titanium and titanium alloys. It has been found that heat treating them in this temperature range severely affects their mechanical properties.
Therefore, it is desirable to deposit adherent noble material on titanium and titanium alloys prior to coating them with ceramics, hard metal and metal compounds and diamond-like carbon. Additionally, it is desirable to deposit adherent noble material on titanium and titanium alloys without exposing them to undesirable high temperatures such as temperatures ranging from 540.degree. to 800.degree. C.
British Pat. No. 1,540,718 discloses a process for the formation of wear resistant hard coating consisting of W.sub.3 C using a mixture of WF.sub.6, benzene, toluene or xylene and hydrogen under sub-atmospheric pressure and temperatures ranging from 350.degree. to 500.degree. C. As that patent points out, it is difficult to obtain good direct adhesion between W.sub.3 C and steel substrates. As a means of overcoming this difficulty, this patent suggests interposing an additional thin layer containing nickel or cobalt between the steel and the W.sub.3 C in order to get a good wear-resistant surface on the steel substrate. British Pat. No. 1,540,718 does not teach a process for coating nonferrous metals and alloys with W.sub.3 C.